1. Field of the Invention:
This invention relates to a blood circulating circuit for a membrane-type artificial lung. More particularly, the invention relates to a blood circulating circuit of the type described for holding the amount of extra-corporeally circulating blood within fixed limits, and to a reservoir for use in the blood circulating circuit.
2. Description of the Prior Art:
The membrane-type artificial lung features less variation in the quantity of stored blood as compared with bubble-type artificial lungs or the like, and makes it possible to gain an understanding of the amount of extra-corporeally circulating blood. However, there has not yet been developed a blood circulating circuit for a membrane-type artificial lung that can perform the foregoing with a high degree of accuracy. The blood circulating circuit of a membrane-type artificial lung generally employs a flexible reservoir for accommodating the blood that flows in an extra-corporeal circulating circuit. A flexible reservoir has the following functions, which will be described later in greater detail:
(1) to accommodate an excess of blood resulting from a variation in the extra-corporeally circulating blood of the patient, and
(2) to prevent the intrusion of air into the circuit in order to protect the patient against blockage of blood vessels when there is an accidental interruption of blood flow into a tube located within a circuit for drawing the blood from the patient.
A blood circulating circuit for a membrane-type artificial lung having a flexible reservoir that possesses the above functions has been disclosed in Japanese Laid-Open Patent Publication No. 52-28195 and has the construction shown in FIG. 1. The membrane-type artificial lung used in this blood circulating circuit is of the stacked or laminated type, wherein the spacing between laminations is regulated by externally applied air pressure. In the case of a single-pump system, pressure applied to the region within the patient's body where blood is returned during extra-corporeal circulation acts upon the membrane-type artificial lung as a restitution pressure, and the spacing between the membrane layers is widened by changes in this pressure and in the pressure that prevails within the circuit, causing separation of seals and the risk of diminished gas-exchange efficiency. With the two-pump system depicted in FIG. 1, on the other hand, it becomes necessary to change the blood delivery pressure at a pump 10, as well as the blood delivery pressure at a pump 11. It is also necessary to alleviate the restitution pressure acting on a membrane-type artificial lung, designated at 14, by providing a reservoir 13. The arrangement therefore includes two reservoirs 12, 13 as well as the two pumps 10, 11. Numeral 15 denotes a heat exchanger, 16 a cardiotomy reservoir, and 17 a filter.
The above arrangement is disadvantageous in that it complicates the main circuit, shown by the bold lines, and in that it calls for an increase in the amount of priming. As shown in FIGS. 1 and 2, each of the venous reservoirs 12, 13 is constricted at its central portion by a clip 18 so that the walls of the reservoir are brought closely together, the arrangement being such that reservoir wall movement is regulated by adjusting the amount of constriction offered by the clip 18. With this design, however the movement of the wall portions not constricted by the clip cannot be regulated. As a result, there is a large variation in the internal volume, so that this scheme is not adequate for measuring the amount of extra-corporeally circulating blood. In addition, as shown by the broken lines in FIG. 2, the clip 18 forms the reservoir into a vessel which is capable of expanding freely in shape. In other words, the reservoir is converted into two interconnected vessels that are deformable or expandable freely in terms of shape, with the clip 18 being located at the center of the arrangement. However, as the venous reservoirs 12, 13 undergo deformation, such as when absorbing fluctuation in the extra-corporeal circulating conditions, it is difficult to gain an accurate understanding of the amount of blood in the circulating circuit. This means that one cannot gauge increases or decreases in the amount of blood within the patient's body, making it impossible to accurately grasp the extent of fluid loss resulting from bleeding or urination. This in turn involves the danger that correct timing for resupply blood and liquid will be lost. A further disadvantage is that the reservoir is compartmented at the portion constricted by the clip, thereby restricting the flow of blood.
As mentioned hereinabove, reservoirs such as those designated at 12 and 13 in FIG. 1 are provided in an extra-corporeal blood circulating circuit to cope with fluctuations in the amount of extra-corporeally circulating blood, that is, to increase the amount of blood delivery when there is a decrease in the amount of blood arriving from the patient, by way of example. To this end, the reservoirs function to retain a certain amount of blood within the circuit at all times. The other reason for providing the reservoirs is to effectively remove air bubbles produced within the blood circuit. Specifically, in open-heart or other surgery, air bubbles may become entrapped in the tube feeding blood from the patient if the catheter for drawing off the blood is inadequately retained within the blood vessel, or when the catheter is withdrawn therefrom. Failure to effect complete removal of air bubbles from the circuit can lead to blockage of small air vessels associated with the brain or other vital organs, thereby causing brain damage and possible loss of life. Although increasing the reservoir capacity provides a corresponding improvement in the air bubble removal effect, the greater capacity necessitates an increase in the amount of circuit priming. The result is an increase in the amount of blood transfused, with a greater possibility of post-operative hepatitis. Another reason for rejecting this expedient is the greater consumption of blood that is required.
One attempt at a solution to the above problem is disclosed in Japanese Laid-Open Utility Model Publication 55-180536. This publication proposes a reservoir in which a screen filter in the shape of a cylindrical body is disposed within a vessel, a blood inlet opens into the lower open end of the cylindrical screen filter, and the upper open end of the cylinder is directed toward a vent or deaeration port provided at the upper portion of the vessel. According to this previously disclosed reservoir, entrapped air bubbles attach themselves to the screen filter and gradually enlarge as more and more of the air bubbles become attached thereto. The air bubbles eventually separate from the screen and are removed by way of the deaeration port. The reservoir therefore exhibits an excellent air bubble removal effect and is also capable of being made small in size. When a large quantity of blood is to be drawn off from the patient, however, the blood within the reservoir develops a strong rotational or vortex-type flow. The disadvantageous result is that air bubbles are drawn into the flow and then pass into the blood outlet along with the blood.